(1) Field of the Invention
The present invention relates to a pixel device of a transflective liquid crystal display (LCD), and more particularly to a pixel device having a hybrid alignment nematic liquid crystal layer driven by a lateral electric field.
(2) Description of Related Art
Along with enormous promotions upon thin film transistor (TFT) fabrication technique, liquid crystal displays (LCD) are broadly adopted to personal digital assistants (PDA), notebooks (NB), digital cameras (DC), digital videos (DV), mobile phones, etc. In an LCD panel, a cold cathode fluorescent lamp (CCFL) is included as a backlight source. The backlight source provides light to pass through layers of optical films including a diffusion film, a polarizer, etc., and to thereafter form a uniform planar image on the LCD.
Generally, most of light emitted from the backlight source is absorbed while passing through the optical films and the liquid crystal panel. As a result, less than 10% of the light can leave the liquid crystal panel to display images. To solve the above-described problem, a reflective LCD introduces ambient light source to replace the CCFL and the related optical films. By adopting the ambient light source, power consumption of the LCD can be reduced and the size and weight of the LCD display can also be minimized. However, visibility of the reflective LCD is poor when the environment is too dark to provide enough ambient light.
In order to overcome the above-described problem, a transflective LCD has been developed by utilizing both a transmission mode and a reflective mode in a single display. The transflective LCD can alternatively use the ambient light or the backlight as its light source. Therefore, in a fair ambient light condition, the ambient light is used to reduce power consumption. On the other hand, in a poor ambient light condition, the backlight is used so as to achieve a better illumination.
FIG. 1 shows a pixel structure of a typical normal-black (NB) transflective LCD. The pixel structure comprises an upper panel 100, a lower panel 300, and a liquid crystal (LC) layer 200 in between. The upper panel 100 has a glass substrate 108 as a main body. A quarter wave plate (QWP) 106 and a first polarizer 104 are stacked on an upper surface of the glass substrate 108, and a color filter (CF) 102 and a common electrode 110 are stacked under a lower surface of the glass substrate 108. The lower panel 300 has a glass substrate 308 as a main body. Another QWP 306 and a second polarizer 304 are stacked under a lower surface of the glass substrate 308, and a reflector 314 for forming a reflector-covered reflective region and a transmission region cover a portion of an upper surface of the glass substrate 308. A pixel electrode 310 as shown is formed over the reflective region and the transmission region of the reflector 314 and also covers the glass substrate 308. The LC layer 200 for imaging is driven by the electric field formed between the common electrode 110 and the pixel electrode 310.
FIGS. 2A and 2B shows operation principles of the transflective LCD of FIG. 1 with no operation voltage. In the reflective region, as shown in FIG. 2A, the ambient light A initially passes through the first polarizer 104 to form a linear polarized light A1. A principal axis of the QWP 106 is arranged to form a 45-degree angle with respect to the transmission axis (shown in a dotted line) of the first polarizer 104, such that the linear polarized light A1 can be converted into a circularly polarized light A2 after penetrating the QWP 106. The circularly polarized light A2 can then pass through the LC layer (not shown in this figure) and be reflected back into the LC layer again by the reflector 314. In this application, because no operating voltage is applied to the LC layer, another circularly polarized light (not shown in this figure) having an opposite orientation with respect to the circularly polarized light A2 can be formed and pass through the QWP 106 to form another linearly polarized light A3. It is noted that the linearly polarized light A1 is perpendicular to the linearly polarized light A3. That is, the linearly polarized light A3 makes a 90-degree angle with respect to the first polarizer 104 and thus cannot penetrate the first polarizer 104.
In the transmission region, as shown in FIG. 2B, the backlight B initially passes through the second polarizer 304 to form a linearly polarized light B1, and then the B1 passes through the QWP 306, the LC layer (not shown in this figure), and the QWP 106. The LC layer does not affect the polarization of the linearly polarized light B1 if no operation voltage is applied, and the fast axes of QWP 306 and 106 are perpendicular to each other to result in zero combined retardation. Thereby, the linearly polarized light B1 is converted into a linearly polarized light B2 with identical polarizing direction. Furthermore, because the transmission axis of the first polarizer 104 makes a 90-degree angle with respect to that of the second polarizer 304, so the linearly polarized light B2 cannot penetrate the first polarizer 104.
FIGS. 3A and 3B shows operation principles of the transflective LCD of FIG. 1 when an operation voltage applied. In the reflective region, as shown in FIG. 3A, the ambient light A initially passes through the first polarizer 104 to form a linear polarized light A4, and then the A4 passes through the QWP 106. For the principal axis of the QWP 106 makes a 45-degree angle with respect to the transmission axis of the first polarizer 104, so the linear polarized light A4 can be converted into a circularly polarized light A5 after leaving the QWP 106. The circularly polarized light A5 then passes through the LC layer 200 but reflected back into the LC layer 200 again by the reflector 314. To optimize reflective displaying brightness, the LC layer 200 is set with quarter-wave retardation by adjusting the operating voltage and the thickness of the LC layer 200 such that the circularly polarized light A5 can be converted into a circularly polarized light A6 with identical polarizing orientation. The circularly polarized light A6 then passes through the QWP 106 and is converted into a linearly polarized light A7 which has an identical polarized direction with respect to the transmission axis of the first polarizer 104. The light A7 can fully penetrate the first polarizer 104.
In the transmission region as shown in FIG. 3B, the backlight B initially passes through the second polarizer 304 to form a linearly polarized light B3, and then the B3 passes through the QWP 306, the LC layer 200, and the QWP 106 to form a polarized light B4. To optimize transmission displaying brightness, the LC layer 200 is set with half-wave retardation by adjusting the operation voltage and the thickness of the LC layer 200. Upon such an arrangement, the linearly polarized light B3 can be converted into the linearly polarized light B4 whose polarizing direction makes a 90-degree angle with respect to that of the linear polarized light B3. Furthermore, because the transmission axis of the first polarizer 104 makes a 90-degree angle with respect to that of the second polarizer 304, so the linearly polarized light B2 can fully penetrate the first polarizer 104.
It should be noted that, in the traditional transflective LCD of FIG. 1, the distances between the pixel electrode 310 and the common electrode 110 of the reflective region or that of the transmission region are the same. Therefore, the LC layer 200 on the reflective region and that on the transmission region is under the same strength of the electric field. Under the condition of optimum reflective displaying brightness, the LC layer 200 is set with quarter-wave retardation, while the LC layer 200 is set with half-wave retardation to optimize the transmission displaying brightness Thus, a compromise should be made between these two optimal conditions for the reflective and the transmission displaying brightness, and also an induced descent on the visibility of the LCD should be resolved.
Accordingly, there is definite a need of providing a pixel device of transflective LCD having different electric field strength in reflective and transmission regions so as to reach an optimal condition for both reflective and transmission displaying brightness.